[cc1] Guard popctx() of dangling pointers in yylval.sym
[cc1] Guard popctx() of dangling pointers in yylval.sym

When a context is popped all the symbols defined in it
are killed. Since the lexer reads a token ahead it
already read some token that maybe is an identifier
defined in the context, and it means that yylval.sym
is an invalid pointer. This new code detects the
situation and relookup the symbol again.

k0gaMSX/scc,k0gaMSX/scc,k0gaMSX/scc